In the prior art of blow molding, there have been proposed a wide variety of core rods for use in blow molding apparatus, such as that disclosed in the copending patent application filed Sept. 25, 1972, Ser. No. 291,651, entitled BLOW-MOLDING PLASTICIZING MANIFOLD. However, such blow cores or core rods have not been entirely satisfactory, often causing the entrapment of air in a parison, resulting in a defective product, sometimes causing overheating or burning of the product, or requiring excessive cooling time to retard output, and presenting other difficulties in practical usage.